There have been known electric power generating devices that use an electret material having properties of semipermanently holding electric charges. Those electric power generating devices include electret electrodes made of an electret material, and counter electrodes configured to face the electret electrodes. Through use of electrostatic induction caused by variation in overlapping area between the electret electrodes and the counter electrodes, electric power is generated. Such electric power generating devices have advantages of being relatively smaller in size and capable of converting oscillation of the electrodes, which is caused in conjunction with movement of the device itself, to electrical energy. Thus, there has been investigated the possibility of application to portable electric apparatus configured to be worn or carried by users, such as a wristwatch.
Normally, in the electric power generating devices, in order to enhance efficiency in generation of electric power, a plurality of electret electrodes are arranged at intervals. A plurality of counter electrodes are also arranged so as to face the plurality of electret electrodes. With this configuration, through movement of the electret electrodes relative to the counter electrodes, electric power can be simultaneously extracted from the plurality of counter electrodes. In Patent Literatures 1 to 4, both the electret electrodes and the counter electrodes are arranged in a circular shape. Through rotation of one of the electret electrodes and the counter electrodes, charging and discharging of the plurality of counter electrodes are simultaneously performed.